Night vision devices are currently an important asset for the warfighter. Their use has allowed soldiers a tactical advantage in low light level (i.e. nighttime) operations and increased their lethality and survivability on the battlefield.
Technology has allowed for many components of these systems to become smaller and lighter. The advance from analog image intensification tubes to digital focal planes has reduced the weight and length of these systems, and plastic optical materials have allowed for lighter eyepiece optics. Upcoming innovations promise to reduce eyepiece size and weight further.
The objective lens has resisted these trends. Plastics are not available as a material for use by the designers and manufacturers of these systems, as the haze and scatter of these materials precludes their use in front of systems that amplify the incoming light. The military has been looking for a method to reduce the length and weight of these lenses, and therefore on the strain that the soldier experiences with prolonged use of these systems.
A challenge in designing lenses for I2 devices with traditional optics has been the requirement for high performing lenses while maintaining a high numerical aperture, or low f-number, in order to gather enough photons from the object scene to maintain a high signal-to-noise ratio. In this case, the f-number for the objective lens is f/1.3. The difficulty in designing high performance systems with these requirements (large field of view, color correction over a larger waveband, and fast f-number) have led to objective designs with a large number of elements in order to meet performance as measured by spot size or modulation transfer function (MTF), a measure of contrast in the system.